1. Field of the Invention
The present invention relates to a numerical control system capable of appropriately controlling communication with communication devices and efficiently using a communication band.
2. Description of the Related Art
FIG. 6 is a block diagram illustrating a numerical control system according to a related art. FIG. 7 is a block diagram illustrating an example of transmission from a numerical controller to amplifiers in the numerical control system according to the related art. As illustrated in FIG. 7, in the numerical control system according to the related art, in communication from the numerical controller 10 to an amplifier, a numerical controller 10 transmits transmission data to all amplifiers in each communication cycle. Further, as illustrated in FIG. 8, in the numerical control system according to the related art, in communication from the amplifier to the numerical controller 10, all the amplifiers transmit transmission data to the numerical controller in each communication cycle.
In an industrial network, it is important to guarantee that communication is necessarily executed (punctuality) within a certain communication period of time. As the punctuality is guaranteed, a network can be applied to an application in which a strict real time property such as synchronous control of motion control is required. In a numerical control system in which the punctuality is guaranteed, a communication period of time, a communication cycle, the number of communication devices, a communication data amount, a distance between communication devices, and the like are specifically decided and managed.
For example, IEEE1394 serving as an international standard is not an industrial communication standard, but isochronous transfer is specified as a method of guaranteeing punctuality of real-time data in which it is problematic if a reproduction operation stops in the middle of reproduction such as an image or a sound (see Japanese Patent Application Laid-Open No. 2002-319942). Specifically, a transfer band is divided into isochronous transfer and non-isochronous transfer in a time division manner, and a decided amount of data is transferred to a decided number of devices during a period of time allocated to the isochronous transfer.
In the related art, in communication in which punctuality is guaranteed, since a communication period of time, a communication cycle, the number of communication devices, a communication data amount, a distance between communication devices, and the like are specifically decided and managed, there is no flexibility in a communication system. As described above, in the communication scheme of the related art, the punctuality is secured in communication between the numerical controller and the amplifier, but since communication cycles are uniformly decided, even when there is an amplifier that can be controlled even in a slow communication cycle, it is necessary to adjust a communication specification according to an amplifier that desires a fastest communication cycle. As a result, the number of connectable amplifiers is limited, and if it is simply assumed that the numerical controller performs direct communication with the amplifiers via a serial bus in a one-on-one manner, and there is neither delay in a communication path nor circuit delay caused as a communication packet passes through each amplifier, the following inequality holds:T×R>N×B 
Thus, N<T×R/B,
Here, T (s) indicates a communication cycle,
R (bps) indicates an effective communication rate,
N indicates the number of amplifiers, and
B (bit) indicates a communication amount per amplifier.
Further, transmission and reception data amount of serial communication is equal to transmission and reception data amount of all amplifiers.
In a system in which a communication rate of serial communication and a communication amount per amplifier are already decided, a communication cycle is in inverse proportion to the number of connectable amplifiers.
As described above, when there is an amplifier that desires a fast communication cycle and a serial communication cycle is made fast according to the amplifier, the number of connectable amplifiers is reduced.